A honeycomb structure is used as a trap filter for an exhaust gas in trapping and removing, for example, a particulate matter (particulates) included in the exhaust gas from a diesel engine or the like, and is incorporated for use as a diesel particulate filter (DPF) in an exhaust system of the diesel engine or the like.
Such a honeycomb structure has, for example, a structure in which a plurality of cells divided by porous partition walls made of silicon carbide (SiC) or the like to form fluid flow paths are arranged in parallel with one another in a central axis direction. Moreover, the ends of the adjacent cells are plugged alternately (in a checkered pattern). That is, one end of one of the cells opens and the other end of the one cell is plugged, whereas one end of another cell adjacent to this one cell is plugged and the other end of the other cell opens.
According to such a structure, the exhaust gas which has flowed into a predetermined cell (an inflow cell) from one end of the cell is transmitted through one of the porous partition walls and is discharged through a cell (an outflow cell) adjacent to the inflow cell. When the exhaust gas is transmitted through the partition wall, the particulate matter (particulates) in the exhaust gas is trapped by the partition wall, whereby the exhaust gas can be purified.
To continuously use such a honeycomb structure (the filter) for a long period, the filter needs to be regenerated. That is, to eliminate the increase of a pressure loss generated by the particulates deposited in the filter with an elapse of time, the particulates deposited in the filter need to be burned and removed. There has been a problem that during this filter regeneration, a large thermal stress is generated to cause defects such crack and destruction in the honeycomb structure. To meet a demand for the improvement of thermal shock resistance against such a thermal stress, there is suggested a honeycomb structure having a divided structure in which a plurality of honeycomb segments are integrally bonded via bonding material layers to impart, to the structure, a function of scattering and relaxing the thermal stress, so that the thermal shock resistance can be improved to a certain degree. In the honeycomb structure having such a divided structure, a plurality of honeycomb segments each having a shape constituting a part of the whole structure are assembled in a direction vertical to the central axis to constitute the whole structure. The honeycomb segments are integrally bonded via the bonding material layers to form a honeycomb segment bonded element so that the whole sectional shape cut along a plane vertical to the central axis is a predetermined shape such as a circular shape. Afterward, the outer peripheral surface of the honeycomb segment bonded element is coated with a coating material.
In recent years, a demand for further enlargement of the size of the filter has risen, and the thermal stress generated during regeneration has increased. To prevent the above-mentioned defects, the improvement of the thermal shock resistance of the structure has strongly been demanded. Above all, it is demanded that in the bonding material layers for integrally bonding the plurality of honeycomb segments, excellent stress relaxing function and bonding strength are realized to realize the honeycomb structure having excellent thermal shock resistance.
To solve such a problem, for example, a honeycomb structure is disclosed in which a bonding material layer material forming the bonding material layer between the honeycomb segments satisfies at least one of conditions that the material has a Young's modulus of 20% or less of that of a honeycomb segment material and that the material strength of the bonding material layer is smaller than that of the honeycomb segment, that is, the bonding material (the bonding material layer material) having a small Young's modulus and configured to relax the thermal stress is used, whereby an only small thermal stress is generated during actual use, and the honeycomb structure has such a durability that any crack is not generated (see Patent Document 1).
Moreover, a honeycomb filter for purifying an exhaust gas is disclosed in which a plurality of columnar porous ceramic members provided with a large number of through holes separated by the partition walls and arranged in a longitudinal direction are bonded together via adhesive layers. An adhesive layer thermal expansion coefficient αL and a porous ceramic member thermal expansion coefficient αF have a relation of 0.01<|αL−αF|/αF<1.0, whereby a local temperature change occurs between the porous ceramic members. In consequence, the generated thermal stress can be relaxed, and any crack is not generated. The filter has excellent strength and durability (see Patent Document 2).
However, the lowering of the Young's modulus of the bonding material (the bonding material layer material) disclosed in Patent Document 1 is effective for relaxing the thermal stress generated during the actual use, but there has been a problem that the lowering of the Young's modulus cannot sufficiently realize the relaxing of the thermal stress generated in the honeycomb structure.
On the other hand, as to the bonding material (the material constituting the adhesive layers) disclosed in Patent Document 2, the thermal expansion coefficient of the honeycomb segment is not equal to that of the bonding material, whereby the generated thermal stress is relaxed. However, even in a case where the thermal expansion coefficient of the bonding material is not equal to that of the honeycomb segment, when the thermal expansion coefficient of the bonding material is higher than that of the honeycomb segment, there is a problem that the thermal stress generated in the honeycomb filter increases. Moreover, when a carbide or a nitride is used in a filler, there is a problem that it cannot be expected that the thermal expansion is lowered sufficiently to relax the thermal stress generated in the honeycomb filter.
Patent Document 1: Japanese Patent Application Laid-Open No. 2001-190916
Patent Document 2: International Patent Application Publication No. 2003-067042